In U.S. Pat. No. 4,143,940 to Khoe a device for coupling a laser radiation source to a monomode optical transmission fiber is disclosed in which the end of fiber's core is covered with a reflecting layer adjoining one cleaved mirror end of the laser. The core's reflecting layer causes a local increase in the reflectivity which causes only the lowest order transversal mode to be stimulated. A high coupling efficiency is achieved which may be increased further by slightly flattening the fiber end so that it has an elliptical cross-section, and by disposing a semi-ellipsoid lens on the end of the fiber core. Care must be taken that the major axis of the ellipse remains smaller than the width of the effective active layer of the laser so that the modeselective effect of the core's reflecting layer is not lost. The lens has a correcting effect on the asymmetrical divergence of the laser beam. This lens is made as a unitary mass with the fiber core in U.S. Pat. No. 4,370,021 to Khoe et al.
For many applications, such as laser surgery, it is desirable to obtain not only the highest possible optical power level, but also a high power density in a small core diameter multimode fiber. Due to effects of divergence of laser light and the mismatch of the guided modes in a rectangular optical waveguide of a diode laser array with those in a cylindrical optical fiber, the coupling efficiency is low. Previously, 140 milliwatts of continuous wave optical power was reported to be transmitted through a 100 micrometer core, 0.3 numerical aperture (N.A.) optical fiber using a laser diode array. The laser array had a lateral dimension of 100 micrometers with a lateral beam divergence of five degrees measured at full-width-half-maximum (FWHM). The beam had a vertical dimension of one micrometer with a 30 degrees FWHM divergence. These characteristics are typical of laser arrays. The coupling efficiency with that fiber was approximately 50 percent.
It would be desirable, however, to couple the same amount of total power into a smaller core diameter optical fiber. This would lead to a higher power density. However, if one simply butt couples the 100 micrometer wide laser array described above to a 50 micrometer core diameter fiber, then much of the laser light would not be guided into the fiber. That is, the coupling efficiency would be quite low.
It is an object of the invention to couple high densities of optical power efficiently between a diode laser array and a small core diameter optical fiber.